Along with weight reduction and size reduction of electric appliances in recent years, a lithium secondary battery having a high energy density is being developed.
An electrolyte solution used in a lithium secondary battery is generally constituted mainly by a lithium salt and a nonaqueous solvent. Examples of the main component of the nonaqueous solvent include a cyclic carbonate, such as ethylene carbonate and propylene carbonate; a linear carbonate, such as dimethyl carbonate and ethylmethyl carbonate; and a cyclic carboxylic acid ester, such as γ-butyrolactone and γ-valerolactone.
In order to improve the characteristics of the secondary battery, such as load characteristics, cycle characteristics, storage characteristics and low temperature characteristics, various investigations have been made for the nonaqueous solvent and the lithium salt.
For example, Patent Document 1 discloses that upon using a mixture of an asymmetric linear carbonate and a cyclic carbonate having a double bond as the nonaqueous solvent, the cyclic carbonate having a double bond is predominantly reacted with a negative electrode to form a film with good quality on the surface of the negative electrode, and thereby a nonconductor film on the surface of the negative electrode due to the asymmetric linear carbonate is prevented from being formed to improve the cycle characteristics.
In a secondary battery using an electrolytic solution containing only LiPF6 as the lithium salt, PF5 formed through dissociation of LiPF6 (LiPF6→Li++PF6−→Li++F−+PF5) breaks the C—O bond of the carbonate to decompose the carbonate (self-discharge), whereby the battery capacity is lowered upon storing. Patent Document 2 discloses, however, that in a secondary battery using an electrolytic solution containing both LiPF6 and LiBF4, an anion (BF4−) formed from LiBF4 suppresses the decomposition of PF6− formed from LiPF6 to stabilize the electrolytic solution, whereby the battery capacity can be prevented from being lowered upon storing. Patent Document 2 discloses the use of a mixture of a cyclic carbonate and a linear carbonate as the nonaqueous solvent, and a mixture of ethylene carbonate and diethyl carbonate is used in the examples thereof.
Patent Document 3 discloses that in a lithium secondary battery using aluminum as a current collector, a high ionic conductivity can be ensured, and aluminum can be prevented from being corroded, by using an electrolytic solution containing a cyclic perfluoroalkylene disulfonylimide salt as the lithium salt. Patent Document 3 also discloses that the cyclic imide salt can be used in combination with other lithium salts, such as LiBF4 and LiPF6, and is preferably used in a proportion of 10 times by mole the other lithium salts.
However, demands for high performance of a lithium secondary battery are being increased in recent years, and such a lithium secondary battery is being demanded that has a high capacity and improved high temperature storage characteristics and cycle characteristics.
As measures for increasing the capacity of the battery, it has been ordinarily employed such a design method that an electrode active substance is packed into the limited battery volume in an amount as large as possible, for example, the electrode active substance is compressed to increase the density thereof. However, another problem arises upon increasing the capacity of the battery. For example, upon decreasing the void in the battery, the battery internal pressure is significantly increased upon forming a gas even in a small amount due to decomposition of the electrolytic solution.
In the case where a battery is used as a backup power source for power outage or a power source for a portable device, a continuous charging method is employed, in which a weak electric current is supplied to the battery to compensate the self-discharge of the battery for the charged state maintained. In the continuous charging method, the electrode is always in a state with high activity, whereby the decrease of the battery capacity is accelerated, and a gas is liable to be generated through decomposition of the electrolytic solution. In a cylindrical battery having a safety valve acting upon detecting abnormal increase of the internal pressure due to overcharge, there are some cases where the safety valve is opened upon forming a large amount of a gas. In a prismatic battery having no safety valve, the battery is expanded or broken in severe cases due to the pressure of the gas formed.
Therefore, a lithium secondary battery is being strongly demanded that gas is suppressed to be formed upon continuous charging, as well as the capacity is suppressed to be decreased.
In the lithium secondary battery disclosed in Patent Document 1 using an electrolytic solution formed by dissolving LiPF6 in a nonaqueous solvent containing (1) ethylene carbonate, (2) methylethyl carbonate and (3) vinylene carbonate, however, there is substantially no effect on preventing decrease of the capacity and on decreasing the gas generation amount upon continuous charging, although the cycle characteristics are improved.
In the secondary battery disclosed in Patent Document 2 using an electrolytic solution containing LiPF6 and LiBF4 as lithium compounds, furthermore, the battery characteristics are deteriorated upon storing under high temperature conditions of 80° C. or higher, and the cycle characteristics are insufficient.
In the secondary battery disclosed in Patent Document 3 using an electrolytic solution containing the particular cyclic imide salt as a lithium compound, moreover, all the storage characteristics, the cycle characteristics and the continuous charging characteristics cannot be maintained simultaneously at high levels upon being produced to have a high capacity, and in particular, the battery characteristics are deteriorated upon storing under high temperature conditions, although aluminum can be prevented from being corroded.
[Patent Document 1]
JP-A-11-185806
[Patent Document 2]
JP-A-8-64237
[Patent Document 3]
JP-W-11-512563